RFR: 7903463: jextract generates empty padding layouts

[Maurizio Cimadamore]

This simple patch fixes a bug where jextract would generate a zero-sized padding layout when handling bitfields. This behavior causes a test failure when running against JDK 21.

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Commit messages:
 - Fix failing test due to zero-size padding layout.

Changes: https://git.openjdk.org/jextract/pull/119/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=119&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903463
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/119.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/119/head:pull/119

PR: https://git.openjdk.org/jextract/pull/119